The way we care for our children affects the size of different bits of their brains in later life, for good or ill. But it also profoundly influences the characteristic patterns of electrochemistry. In particular, if we give them a stressful time, depending on the form it takes, they can have elevated or lowered levels of the hormone cortisol. This is the chemical that prepares us for fight or flight in the face of threat. As children, if we have constantly felt in danger of attack or rejection or abandonment, eventually the system may just shut down. We have permanently low cortisol levels, blunted by habituation to risk.

As adults it may mean we are depressed or prone to be very unresponsive to our surroundings. At its most extreme, we occupy a state of flat emptiness so that a mad axeman could run into the room and we would not be startled or react.

Alternatively, if the childhood stress is more acute, with sporadic occasions of severe maltreatment, cortisol levels are jammed high. In this sort of household, you might be lashed out at by parents or siblings for no reason, witness sudden outbursts of extreme parental discord, perhaps be suddenly dumped with complete strangers before you are old enough to cope.

As adults, that makes us hyper, perhaps with poor concentration and a tendency not to be able to sit still and relax. Overly sensitive, someone dropping a teaspoon or just making a harmless remark could lead to a massive overreaction, possibly including violence.

However, the good news is that in most cases, once the cortisol level has been set, like a thermostat, it can still be changed by subsequent experience.

Studies of extreme cases illustrate the basic cause of abnormal cortisol levels. In three studies of children in orphanages where care entailed institutionalised neglect (no one-to-one care, left to cope on their own for hours on end, in some cases from soon after birth), all the children had blunted responses to threat and low cortisol.

But they were not permanently jammed low. Tested three years later, after adoption into good homes, not one had low cortisol. Furthermore, in studies where the levels are measured several times immediately after adoption and in succeeding months, it seems that the better care steadily raises the cortisol levels to normal ones, achieving this fully around the eighth month.

It seems to be only the most extreme cases that are hard to reverse, where the child was severely maltreated for a prolonged period in their original family. In studies of families with exposure to parental violence, threats of abandonment and in which attempts by the child to express affection are rejected, children's cortisol levels are sky high. Sadly, adoption and good quality care does not completely normalise the levels even years later.

Whether too high or too low, abnormal cortisol levels damage growth of key regions of the brain and are associated with a host of psychiatric problems. Unfortunately, it is not just care provided in severely dysfunctional families that can cause abnormal levels, much commoner problems and practices can also do so. There is widespread ignorance of this evidence, even among educated parents. For example, as I shall explore in next week's column, common problems in the early years such as parental depression or inadequate substitute care cause abnormal cortisol. While realising this may be worrying for parents, the upside is that you can significantly reverse the harm by changing the pattern of care. If the harm is already done and the child is in middle childhood you can still achieve a great deal by giving the child extra love.